The field of the invention is felts for use in a papermaking machine, and more particularly, press felts for use in the press section of a papermaking machine.
In the conventional papermaking process, a water slurry or suspension of cellulose fibers, known as the paper "stock", is fed onto the top of the upper run of a traveling endless forming belt. The forming belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the drainage side of the fabric.
After leaving the forming medium, the somewhat self-supporting paper web is transferred to the press section in the machine and onto a press felt, where still more of its water content is removed by passing it through a series of pressure nips formed by cooperating press rolls, these press rolls serving to compact the web as well. It is this press felt which is the subject of the present invention.
Subsequently, the paper web is transferred to a dryer section where it is passed about and held in heat transfer relation with a series of heated generally cylindrical rolls to remove still further amounts of water therefrom.
Ideally, press felts should have at least the following properties. First, they should have a top surface that is fine enough to produce a smooth finish and minimize marking of the sheet of paper being produced. Second, they should be open enough to allow water to drain through without significant impedance. Third, the felts should be resilient enough to quickly recover from repeated high nip pressures over a long period of time. Fourth, they should be tough and strong enough to provide good stability, wear resistance and felt life.
The base fabric layer of the press felt can be woven or nonwoven. The woven base fabric may be a single layer, a double layer, a triple layer fabric, or a laminated fabric. The term "single layer" as used herein refers to a fabric comprising one set of machine direction yarns and one set of cross machine direction yarns. "Double layer" refers to a fabric comprising two sets of cross machine direction yarns interweaving them. "Triple layer" refers to a fabric comprising two complete weaves. Each weave includes one set of machine direction yarns and one set of cross machine direction yarns. A thread or threads interweaves the two weaves to produce the triple layer fabric. A laminated base fabric is comprised of two or more base fabric layers.
In felts having a base fabric assembly with two or more layers of woven fabric, the top layer can be woven fine to prevent marking from the coarser machine side bottom layer and to provide good retention of the needled fibers. Also the top woven fabric can be more compressible than the bottom layer. The other layer, the bottom base fabric layer, can be made relatively coarse so that it has a high void volume and a high degree of compaction resistance and wear resistance.
Generally, press felts are assembled in the following manner. If the base fabric is not woven endless, the ends are joined by stitching a seam in a conventional manner. The base fabric is then installed on a needle loom. Where multilayer base fabrics are employed, the fine layer comprises the outside or top loop. Batt fibers are applied to the top side or paper contacting surface of the base fabric layer of the press felt, in sufficient quantity and weight to give good bulk and cushion properties. The fibers are anchored to the base fabric layer by one or more needling operations. Thus, the surface of the press felt which contacts the paper web is a felt, formed as the batting material fibers are needled to the base fabric. Fibers may also be needled to the bottom of the woven base fabric to ensure good anchoring of the fibers on the top side.
Needling the entire structure gives the felt a uniform thickness. Needling also provides a cushioned absorbency to the felt and distributes the pressure uniformly across the width of the felt for efficient water removal. Both uniform thickness and pressure distribution help to reduce vibration of the press rolls of the papermaking machine. Needling is necessary to compress the felt to a given density and resiliency and to entangle the fibers in the base fabric so they do not come loose during the papermaking operation.
The batt material may be made up of fibers of any of a number of well known compositions, including natural fibers such as wool, but preferably will be made in whole or in part from synthetic materials such as nylon, dacron, etc. In this connection, it is desirable that these fibers be relatively coarse or of large diameter. They will be selected for their compaction resistance, that is, their tendency to resist bending or deformation at fiber cross-over points since this enhances their ability to produce a good papermaking surface.
It is desirable to have a felt which is soft and easy to bend, since a certain amount of bunching is required in order to install the fabrics on the machine. A problem with existing felts is that they tend to be rather stiff and inflexible, which makes installation on the papermaking machine difficult.
It is also desirable to have a papermaking fabric with optimum drainage characteristics. The drainage characteristics of a papermaking fabric greatly affect the quality and type of paper produced on that fabric. These characteristics include the amount of void space per surface area, the amount of void volume per volume of fabric, the average area of a clear path straight through the papermaker's fabric, and the relative percentage of fabric area contacting the paper as it relates to the total area of the paper. The desired press felts are woven so as to have relatively large open areas or voids which will enhance the fabric's water conveying capabilities so that the water may be removed from the felt upon passage over a suction box.
It is also desirable to a have felt which aids in minimizing vibration problems related to the pressing of water from the web. The press section of a papermaking machine includes transport rollers that move the papermaking fabric along, with the paper web supported above the fabric. Above the paper web are arranged several press rolls. An upper press roll is located opposite a cooperating roll located below the papermaking fabric. The purpose of the press roll is to press down upon the wet web and squeeze water out from the paper and into the fabric. The rolls press down with a force of about 700 pounds per lineal inch. Thus, for a roll forty feet wide, the roll experiences 336,000 pounds of force. At the same time, the paper moves past the roll at 1,000 meters per minute. Thus, the press roll is spinning at a high rate, under an extreme load. The roll is supported from above by a frame, with shock absorber type elements interposed between the roll and the frame. However, any slight imbalance in the roll, which invariably arises, causes the press roll to shake and vibrate. Due to the extreme forces, this vibration causes flattening of the roll, which further exacerbates the vibration. Eventually, the roll vibrates to such a degree that the entire portion of the building housing the press section quakes.
Aside from the obvious noise pollution and physical discomfort, a wildly vibrating roll presents several other problems. First, paper manufactured with such a roll is irregular and cannot be used for fine applications. Further the vibration of the roll ruins the rolls, which must be removed and reground to return to a balanced state. Similarly, the vibration loosens many of the components in the papermaking machine itself. The machine then must be frequently serviced. Finally, the vibration causes the papermaking fabric to compact to a certain degree, such that it no longer exhibits the desired drainage and vibration accommodation characteristics.
Thus, it is an object of the present invention to provide a papermaker's fabric which is easier to install on the papermaking machine.
It is another object of the invention to provide a papermaking fabric which is softer and easier to bend than conventional fabrics.
It is another object of the present invention to provide a papermaker's fabric having enhanced capacity for removing water from a paper sheet.
It is another object of the invention to provide a papermaker's fabric which has greater void for drainage.
It is another object of the present invention to provide a papermaker's fabric which has optimum bulk and weight.
It is another object of the present invention to provide a papermakers' fabric for the press section of a papermakers' machine which minimizes the vibration of the press roll.